The present invention relates generally to fiber optic devices, and more specifically to fiber optic switches based on directional couplers.
The prior art reveals a number of approaches to switching light in one optical fiber to a selected one of a plurality of other optical fibers. U.S. Pat. No. 4,329,017 to Kapany et al. shows a movable concave reflector that focuses light emerging from the end of an input fiber onto the end of a selected one of a number of output fibers. U.S. Pat. No. 5,028,104 to Kokoshvili shows how such a switch can be configured as a bypass switch. U.S. Pat. No. 5,037,169 to Chun shows a different approach, using a Fabry-Perot etalon filled with a semiconductor medium whose index of refraction can be changed by optical, electrical, or thermal techniques, whereupon the incident light is reflected to one port or transmitted to another, depending on the state of the etalon material.
Yet another approach uses an evanescent-wave directional coupler as a switch (H. Berthou and L. Falco, "Switching Characteristics of a Piezoelectrical Actuated Evanescent-Wave Directional Coupler," Electronic Letters, Vol 23, No. 9, pp 469-470 (Apr. 23, 1987). In this type of switch, a fiber segment is glued in a variable-width V-groove in a silicon substrate. The fiber is then tangentially ground and polished to define an intermediate portion of greatly reduced cladding thickness. The polished fiber portion extends about 100 nm above the silicon surface. Two of these substrate assemblies are spaced apart with the polished fiber portions facing, and the substrate assemblies are flexed to bring the polished fiber portions together. When the polished portions are brought into contact, light traveling in one of the fibers is coupled into the other by the mechanism of evanescent wave coupling. The coupling ratio depends on the distance between fibers, the length of the contact region (the interaction length), the thickness of the cladding in the interaction region, the fiber parameters, and the wavelength of the light. A detailed discussion of the operation of fiber directional couplers can be found in Michel J. F. Digonnet and Herbert J. Shaw, "Analysis of a Tunable Single Model Optical Fiber Coupler," IEEE Journal of Quantum Electronics QE18, 4, 746-754 (April 1982); and Michel Digonnet and H. J. Shaw, "Wavelength Multiplexing in Single-Mode Fiber Couplers," Applied Optics 22, 3, 484-491 (Feb. 1, 1983).
Although the directional coupler approach to switches is potentially fruitful, a number of serious technical problems come to mind. First, the coupler is characterized by a non-negligible degree of cross-talk, both as an inherent matter and as a result of small differences in the coupling from one switch closure to the next. Additionally, the coupling is wavelength sensitive, so a switch that operates at one wavelength is unlikely to operate satisfactorily at a different wavelength. 1308 nm and 1550 nm are two standard wavelengths for fiber optic communications systems, and it would be desirable if a single switch could work for both. Indeed, both wavelengths are often present simultaneously in a multiplexed system. A further problem is that the fabrication tolerances are typically very tight, and minor dimensional variances are likely to compromise performance significantly.